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Penis worms, anuses, and evolution

Now that I have your attention, what is a penis worm? Technically, they are invertebrate animals called priapulids which, if you know your Latin, has the word “penis” in there. What exactly are they? They are sausage-shaped, segmented worm-like animals with an extensible, toothy proboscis they use to capture prey. Here is what a priapulid looks like:

A priapulus “worm” — the proboscis is pointing up in this picture.

What do these weird-looking and rather unknown animals have to do with evolution? A whole lot, as it turns out.

The group of creatures we call Animals is incredibly diverse, and most of us have only a passing familiarity with a small fraction of these denizens. It turns out that once you get past sponges, jellyfish and their kin, and a weird band of animals called comb jellies, there is a great clade of animals called the Bilateria. Sounds formal, but basically the name means that this group of animals is bilaterally symmetrical – that is, these animals have symmetry about their midline and have right and left sides. Look in the mirror – congratulations, you, too, are a member of Bilateria!

The diversity of Bilaterian animals.

You may not appreciate it, but one of the key developments of bilaterian animals is forming a distinct mouth and anus. This involves the formation of a gut tube, and like all tubes it has to start somewhere. That somewhere is a puckered indentation called a blastopore that forms early during their embryonic development. Yes, even we humans develop a blastopore as the beginning of our gut tract.

Among the bilaterian animals, there has traditionally been a split proposed that divides these organisms into two groups based on a fundamental difference in the way their digestive tracts develop. In one group called Protostomes, the blastopore becomes the mouth, and the gut tract develops until it “punctures” the other end of the animal, forming the anus. The word Protostome means “mouth first.” Protostomes include a huge variety of animals such as insects, crustaceans, earthworms, mollusks, and most other “creepy-crawlies” you are familiar with.

In the other group, called Deuterostomes (meaning “mouth second”), the blastopore becomes the anus and the digestive tract stretches from hind to fore, eventually “punching” through the head region to form the mouth. As I like to tell my students, Deuterstomes develop from the bottom up – you may now groan. Something that may make you groan all the more is the fact that we vertebrates are members of the Deuterostomes.

The development of the mouth and anus in Protostomes and Deuterostomes.

What does all this have to do with Priapulids? Everything. You see, the big evolutionary question is which came first, Protostome development or Deuterostome development? Which is the original condition in the common ancestor? Knowing this would inform our understanding of how other changes in development downstream from this evolutionary event were effected, and what we should predict to see in various animal lineages.

Martin-Duran and colleagues (2012) suggest that we have it wrong when it comes to these evolutionary divisions of animals. It turns out that we may be too hung up on what the blastopore forms. Instead, Martin-Duran et al. (2012) suggest that it is the separation of the mouth from the anus that is the major adaptation to focus on in Bilaterian animals. Given that a number of Protostome animals are similar to priapulids in having all the protostome characteristics except that their blastopore forms the anus, it seems that the Deuterostome condition was the most primitive. In other words, the Protostome condition is derived, and probably developed independently several times.

What does all this mean? It means that we now need to re-explore animal relationships and the fossil record to re-test whether the way we understand the evolution of the major animal groups is in need of re-tooling. Who would have thought that a penis worm and the origin of its anus would have such an evolutionary significance?